Linear accelerator (LINAC) relative dosimetry (beam scanning) involves acquiring a measurement of the radiation beam with a field detector and typically a reference detector. The measurement is acquired as two parameters are varied: the field detector's position and time. Using the measured signal from the field detector, a plot of the LINAC's radiation beam intensity versus position within that beam is possible, with an example shown in FIG. 1.
Beam scanning often requires normalizing the measured signal from a field detector by the measured signal from a reference detector. This occurs because the output of the LINAC may change with time, sometimes abruptly. For example, an increase in the LINAC radiation dose rate during a scan results in an increase in the field detector's measured signal. If only a field detector is used, the beam scan will show a change in the beam intensity at that point in space that corresponds to when the LINAC's radiation output changed. This irregularity in the measurement is not indicative of the actual relative dose intensity of the radiation beam. In FIG. 2, a repeat of the scan from FIG. 1 is shown, but in which the LINAC's dose rate changed (increased) during the scan. This change caused the marked change in the amplitude of the beam's relative dose intensity.
Small changes in LINAC dose rate do not affect the radiation therapy treatment. The change in the dose rate effectively changes the amount of time in which the radiation dose is delivered, which is not important to the treatment parameters being measured. The more important parameter during beam scanning is the relative dose intensity of the beam. Using only a field detector may misrepresent this characteristic of the LINAC's radiation beam, as in FIG. 2.
To correct for changes in a LINAC's dose rate, a reference detector is used to normalize dose rate changes. For example, referring to FIG. 3, a radiotherapy system 110 includes a reference detector 120 that is stationary and located in the radiation beam 124. Considering the above example, when the LINAC 112 dose rate increased, it increases the measured signal of the field detector 116, which would otherwise misrepresent the relative dose intensity of the radiation beam 124 (as in FIG. 2). However, the increase in the field detector's measured signal should be matched by a proportional increase in the reference detector's measured signal. A ratio of the field detector's measured signal to the reference detector's measured signal corrects for the changed dose rate that occurred during a scan. The measurement would then match that of FIG. 1, correctly representing the beam's relative dose intensity. While this approach to beam measurement normalization is useful, further improvements are possible.